Network node elements are known on the level of the Network Layer, or Layer 3. Such a network node element has at least three interfaces, at which data packets are exchanged bidirectionally with network elements of a packet-switched network (for example computer systems or “hosts”) and/or with other network node elements.
Such types of network node elements are also referred to in professional circles as “routers”. Depending on the application of the network node element, further functions are added, with the result that in addition to the designation “router” further designations exist for specifically embodied network node elements. For reasons of a simple description the term router is used in the following without excluding further specifically embodied network node elements through this and without restricting the universality of the network node element described.
In a router, essentially two processes are brought to execution. A first process accepts a data packet arriving at an interface of the router and determines an interface of the router to which the data packet is forwarded. This first process is called forwarding. In order to determine an interface to which the incoming data packet is forwarded, a routing algorithm is used which, on the basis of entries in the data packet and on the basis of information maintained in the router regarding the environment of the router, determines an interface for the outbound data packet.
A second process is used for managing the routing algorithm with the objective of optimizing the algorithm to the effect that the choice of the interface for the outbound data packet is as efficient as possible in terms of a short path in the packet-switched network. In addition to the path determination (“routing”), which should be as short as possible, further criteria are added, to which the routing algorithm is normally to be optimized.
One form of managing the routing algorithm which is well-established today is also referred to as “adaptive routing”. In the case of adaptive routing—in contrast to a static routing process—changes in the data traffic and in the topology of the packet-switched network are taken into consideration for an adaptation of the routing algorithm. To this end the router communicates—by means of the aforementioned second process—with neighboring routers in order to determine their connection status and to determine a “metric” for optimizing the routing algorithm. In this situation, the best possible paths are entered in a routing table, while link-state or topological databases contain information relating to the connection status and to the environment of the router. In a general manner, the concept of the routing table is often used for a combination of the aforementioned functional units routing table, link-state and/or topological databases.
The metric, also referred to as “interface metric”, provides a measure for determining the most efficient path. With the aid of the metric, the routing algorithm determines whether one choice of an interface of the router—and thus a chosen path—is more efficient in comparison with another. In the case of a plurality of possible interfaces, it is customarily the interface that exhibits the smallest metric which is selected. The interface with the smallest metric is for example that interface for which a resulting path has the minimum distance to the destination system. In addition to the distance, further criteria can be incorporated into the metric, such as for example the bandwidth which can be achieved on a path, the delay to be expected on a path, the number of network node elements situated on the path (“hop count”), etc.
With regard to a path determination for data packets through networks, any failure of sections on the path to the destination is countered by the dynamic routing described above. However, if the first router on a path fails, then this fault can often not be cleared by means of dynamic routing.
One reason for such an inevitable gap consists in the fact that one router, which guarantees communication with other (partial) networks at the edge of the partial network, is normally noted as the direct “contact partner” for a host or a plurality of hosts. The address of this router is normally noted statically in the respective host. The corresponding router is often referred to as the “default gateway”.
Unless additional measures are taken, following a failure of the first router, or default gateway, the affected hosts would be cut off from any communication extending beyond the partial network, even if still active routers were available in the same partial network.
In order to solve this problem, protocols for increasing availability while using redundant default gateways have been proposed, which are classified as a protocol family FHRP (First-Hop Routing Protocol). The HSRP protocol (Hot Standby Router Protocol) from the company Cisco Systems Inc., San Jose, Calif., USA, makes provision for combining a plurality of routers to form a logical group which is addressed over the packet-switched network as a logical default gateway. The alternative protocols VRRP (Virtual Router Redundancy Protocol) and also GLBP (Gateway Load Balancing Protocol) pursue the same objective with similar methods.
The aforementioned protocols for increasing availability react to a failure of a router within a redundant combination of routers by switching to a different router, whereby the address of the logical default gateway remains unchanged.
In the event of a failure of a first router, which was operating up to that point as the default gateway, a second router assumes the role of the logical default gateway. To this end, a virtual IP address (Internet Protocol) and a virtual MAC address (Media Access Control) of the first router are transferred to the second router which thus assumes the function of the logical default gateway. By transferring the MAC address and the IP address to the second router, this means that a particular host in the partial network can still use the address, noted statically in the host, of the logical default gateway in order to address the logical default gateway, even though after the failure of the first router the second router henceforth assumes the function of the logical default gateway. The hosts do not therefore need to undertake any updating of their addresses maintained in a respective ARP cache (Address Resolution Protocol).
Although within the partial network the course of action described above involving the use of protocols for increasing the availability of the default gateway guarantees a connection beyond the boundaries of the partial network into other networks, it does however result in the fact that the first router relinquishing the role of the default gateway does not take the switch of default gateway into consideration in all interfaces. This can result in communication connections from another (partial) network continuing to be routed by way of the first router, which however is unable to establish any connection with its associated host.